Thyrotropin-releasing hormone and epidermal growth factor regulate iron-regulatory protein binding in pituitary cells via protein kinase C-dependent and -independent signaling pathways

Intracellular iron homeostasis is regulated, in part, by interactions betwe en iron-regulatory proteins (IRP1 and IRP2) and iron-responsive elements (I REs) in ferritin and transferrin receptor mRNAs. In addition to iron, cellu lar oxidative stress induced by H2O2, nitric oxide, and hypoxia, and hormon al activation by thyroid hormone and erythropoeitin have each been shown to regulate IRP binding to IREs. Hormonal signals, in particular mediated thr ough protein kinase C (PKC), play a central role in the modulation of IRP/I RE interactions since phorbol esters were shown to activate IRP binding (Ei senstein, R. S., Tuazon, P. T., Schalinske, K. L., Anderson, S. A., and Tra ugh, J. A. (1993) J. Biol. Chem. 268, 27363-27370). In pituitary thyrotroph s (TtT97), we found that thyrotropin releasing hormone (TRH) and epidermal growth factor (EGF) increased IRP binding to a ferritin IRE, dependent on P KC and mitogen-activated protein kinase (MAPK) activity. In contrast, TRH a nd EGF decreased IRP binding in pituitary lactotrophs (GH3), despite activa tion of PKC and MAPK. IRP1 and IRP2 levels remained constant and IRP2 bindi ng was predominant throughout. TRH and EGF markedly decreased IRP binding i n MAPK kinase inhibitor-treated GH3 cells, whereas, they increased IRP bind ing in phosphatase inhibitor-treated GH3 cells. IRE-dependent CAT reporter translational expression closely reflected IRP binding to the ferritin IRE in both GH3 and TtT97 cells. Interestingly, ferritin protein levels were re gulated similarly by TRH in both cell lines. These data link two different cell receptor systems to common signaling pathways that regulate IRP bindin g and ferritin expression. Remarkably, for TRH and EGF, these effects may b e PKC-dependent or -independent determined by the cell type.